Controlling the dynamics of colloidal particles by critical Casimir forces
Alessandro Magazzù, Agnese Callegari, Juan Pablo Staforelli, Andrea Gambassi, Siegfried Dietrich & Giovanni Volpe
Soft Matter (2019)
Critical Casimir forces can play an important role for applications in nano-science and nano-technology, owing to their piconewton strength, nanometric action range, fine tunability as a function of temperature, and exquisite dependence on the surface properties of the involved objects. Here, we investigate the effects of critical Casimir forces on the free dynamics of a pair of colloidal particles dispersed in the bulk of a near-critical binary liquid solvent, using blinking optical tweezers. In particular, we measure the time evolution of the distance between the two colloids to determine their relative diffusion and drift velocity. Furthermore, we show how critical Casimir forces change the dynamic properties of this two-colloid system by studying the temperature dependence of the distribution of the so-called first-passage time, i.e., of the time necessary for the particles to reach for the first time a certain separation, starting from an initially assigned one. These data are in good agreement with theoretical results obtained from Monte Carlo simulations and Langevin dynamics.
Light-driven Assembly of Motile Colloidal Clusters from Immotile Building Blocks Falko Schmidt, Benno Liebchen, Hartmut Löwen & Giovanni Volpe
APS March Meeting 2019, Boston, USA
6 March 2019 at 8:36-8:48 a.m., Room 258B
Active matter, consisting of self-propelled units locally injecting energy into the system, opens new horizons for the creation of functional soft materials with designable properties. Experiencing a constant energy input, allows active matter to self-assemble into phases with a complex architecture and functionality such as living clusters which dynamically form, reshape and break-up but would be forbidden in equilibrium material by the entropy maximization (or free energy minimization) principle. The challenge to control this active self-assembly has evoked widespread efforts typically hinging on an engineering of the properties of individual motile constituents. Here, we provide a different route, where activity occurs as an emergent phenomenon only when individual building blocks bind together, in a way which we control by laser light. Using experiments and simulations of two species of immotile microspheres, we exemplify this route by creating active molecules featuring a complex array of behaviors, becoming migrators, spinners and rotators. The possibility to control the dynamics of active self-assembly via light-controllable nonreciprocal interactions will inspire new approaches to understand living matter and to design active materials.
Encoding mechanical information through multimolecular structures: Lessons from directed cell migration Seminar by Vinay Swaminathan
from Wallenberg Center for Molecular Medicine Fellow, Lund University
Interactions between cells and their mechanical environment is a critical regulator of important physiological functions that gets hijacked in diseases such as cancer. One such function is directed cell migration where cells sense physical cues such as stiffness, varying topologies and fluid flow and migrate directionally in response. While we know relatively a lot about how individual molecules respond to forces, we still lack the understanding of how these “force-sensitive” proteins come together to function in a cell to allow it to sense and transmit mechanical information critical for function. In this seminar, I will first discuss the mechanical cues a cell in our body encounters and their role in normal physiology and disease. I will then introduce the primary subcellular structure that mediates interactions between the cell and its mechanical environment- Integrin-based focal adhesions. I will then describe in detail my recent findings on how integrin receptors come together in focal adhesions and act as mechanical compass where the 3-dimensional orientation of these receptors is sensitive to the magnitude and directionality of mechanical information and thus encodes it. I will also describe the unique microscopy technique that facilitated the discovery of this novel molecular organization and bridges the gap between crystal structures and resolution-limited microscopy. I will conclude by looking ahead to what such an organization of molecules tells us about building mechano-sensitive structures in cells, how we can test its role and perturb it during cell function and how this will provide us with novel insights into diseases such as cancer and immune-disorders.